Basic Commissioning of a Public Safety BDA with a Spectrum Analyzer
The majority of public safety BDA’s provide signal readings/RF measurements in the graphical user interface (GUI), and some are also equipped with spectrum analyzer software. Although, these readings and software are helpful for general BDA setup and troubleshooting, sometimes it does not provide the level of detail required to fully commission a BDA system, especially when it comes to interference, noise, and troubleshooting.
This Tech Brief will address the general terminology and basic usage of a spectrum analyzer, along with measurements required to commission a BDA system.
Terminology & Settings:
- Frequency: Depending on spectrum analyzer model, the center frequency and span can be specified and/or the start/stop frequencies can be entered.
- Reference Level: Represents the maximum expected amplitude at the input of the spectrum analyzer and is the top edge of the measurement range. In most cases, the maximum expected signal should be slightly below this level.
- Sweep: The length of time it takes the detector to sweep from the start frequency to the stop frequency.
- Resolution Bandwidth (RBW): RBW is a bandpass filter applied to the input signal/monitored frequency range on the spectrum analyzer. The narrow RBW will provide a more detailed spectrum analyzer output, but with a disadvantage of a longer sweep time. If the RBW is too wide, the individual traces of two frequencies could easily be overlapped and appear as one trace, making it more difficult to tell them apart, but with the advantage of a faster sweep time.
The below figure is a comparison between wide RBW and narrow RBW
Figure 1 - Wide RBW Setting vs. Narrow RBW Setting Example
The below figure shows spectrum analyzer results between wide RBW and narrow RBW settings:
Figure 2 - Wide RBW Setting vs. Narrow RBW Setting Results
NOTE: The wider RBW filter also has an increased noise floor as compared to the narrow RBW setting.
- Video Bandwidth (VBW): VBW is a lowpass filter applied to the input signal/monitored frequency range on the spectrum analyzer. The purpose of this filter is to smooth the noise inherent at in the input signal. The smaller the bandwidth, the less noise there will be in the output signal, but this will also increase the sweep time. It is suggested, when working with Public Safety narrowband signals (6.25kHz or 12.5Khz) to set your RBW to 10kHz and the VBW to 1/3 of RBW. This will help distinguish between close channels, and find any low noise, plus makes it easier to compare with FCC requirements. Detector Types: This setting will adjust how the amplitude in each bin/bucket (each sweep contains multiple bins/buckets depending on settings) will be calculated. Eg.- Positive Peak, Negative Peak, Sample Average, etc. For more information, please refer to the spectrum analyzer user manual.
- Pre-Amplifier: When the pre-amplifier is utilized, it will increase the system sensitivity, allowing weaker signals to be measured, thus displaying a more accurate noise floor and Signal/Noise ratio. The rule-of-thumb for the signal input into the spectrum analyzer when the pre-amplifier is being used is < –40dBm. Please refer to the spectrum analyzer user manual for specifics.
- Trace Control/Types: The behavior of the trace(s) can be controlled over a series of acquisitions. Most spectrum analyzer have the option of: Clear & Write, Max Hold, Min Hold, Average. Please refer to the spectrum analyzer user manual for specifics.
- Dynamic Range: This is a characteristic of a spectrum analyzer and good to know when performing measurements and setup. The dynamic range is the maximum power ratio (in dB) between a high-power signal and low-power signal that are present at the input of the spectrum analyzer – such that both signals can be observed and measured at the same time or on the same display. It’s important when measuring noise or noise floor to set the reference level accordingly. Please refer to the spectrum analyzer user manual for specifics.
Now that the general terminology and basic setup have been discussed, it is time to conduct the signal measurements. The below examples use a Signal Hound SA44B spectrum analyzer, but all other manufacturers should have the same minimum features.
- Donor Signal: A donor signal reading should have been performed prior to or during the design phase. When commissioning the BDA, it is necessary to verify the previous measurements or estimations to determine how much BDA DL gain will be required or if a front-end attenuator is required to protect the BDA from being overdriven.
Steps (Figure 3 below):
- Connect the spectrum analyzer to the donor antenna/coax
- Set Control Channel/Frequency and Span in the spectrum analyzer
- Set the Reference Level to slightly higher than the expected input power and adjust, if necessary, during measurement
- Set RBW/VBW
- On a conventional public safety system (no control channel) set the Trace temporarily to Max Hold to capture a DL Signal (voice channel) on the spectrum analyzer
- Set the Marker(s)
- Record the measurements
Figure 3 – Spectrum Analyzer Donor Signal Setting Example
RF Environment & Composite Input Power: It is highly recommended to check the RF environment and measure the composite input power to the BDA prior to commissioning. This should be done for 700MHz and/or 800MHz bands (including ESMR band). More information can be found on this topic by reading here. In the example below, we are concentrating on the 700MHz Band. Most 700MHz BDA’s support the FirstNet Band, but it is not usually required to be retransmitted in the DAS solution. A nearby FirstNet site can cause BDA interference issues if not properly identified prior to the BDA commissioning phase.
Steps (Figure 4 below):
1. Change Frequency (Center frequency = 766.5Mhz) and Span from previous setup and widen the band to cover at least 758Mhz – 775Mhz (preferably a bit wider).
2. Enable Channel Power and set a width of 17Mhz
3. Adjust the Reference Level, if necessary
4. Record the Channel Power
Figure 4 – Spectrum Analyzer RF Environment and Composite Input Power Setting Example
The grey area in the image above indicates a -35.29dBm composite power between 758MHz - 775MHz band. This is considered a high composite input power which is close to the limits of the BDA’s AGC specifications. If the donor signal is low, an external attenuator cannot be applied in this situation, thus an external filter, like the Comba FP-78-IN2, is recommended to reject the FirstNet band. The new Comba NG 700/800 BDA will have an internal duplexer filter option to mitigate FirstNet or 800MHz ESMR issues, thus no longer requiring external BDA filtering.
Isolation Measurement: Comba BDA’s have built-in software to test isolation, but in some situations a manual isolation test is required. This test would also require a signal generator. The setup for the spectrum analyzer would be similar to measuring the donor signal. For information about manual isolation test, please read here and here.
UL Input Measurement: This will require a minimum of three (3) measurements: general noise input from the passive DAS, Maximum UL Signal Input, and Minimum UL Signal Input.
Passive DAS Noise Procedure (Figure 5 below):
1. Connect the spectrum analyzer to the passive DAS
2. Set the spectrum analyzer to the UL frequency band
3. Set the Reference Level low enough to measure the actual noise floor, but not so low as to be limited by the dynamic range of the spectrum analyzer
4. Verify the Pre-Amp is “ON” to increase the sensitivity of the spectrum analyzer
5. Verify if any high noise that falls into the proposed passband or into the programmed filters are identified, as they will be retransmitted and can cause issues at the donor site
6. Additional troubleshooting may be necessary if excessive noise is discovered
Figure 5 – UL Input/Passive DAS Noise Example
Minimum and Maximum UL Input Signal Procedure (Figure 6 below):
To accurately determine UL BDA Gain or if any UL output attenuation is required, measure the Minimum and Maximum UL Input Signals. This can be done by utilizing the Trace feature Max Hold. In addition, if the Max UL input signal is above -30dBm, Comba recommends applying an internal SMA attenuator to the UL path.
Figure 6 – Minimum and Maximum Input Signal Examples
For example, if +8dBm Power/Channel out of the donor antenna is required to reach the donor site with the proper signal level, a UL gain of 65dB is needed in the BDA (assume Donor Antenna Gain ~ 11dB and Cable Loss ~ 3dB).
The formula is: +8dBm = Minimum Input Signal + BDA UL Gain – Cable Loss + Donor Antenna Gain = -65dBm + 65dB – 3dB + 11dB
Verifying BDA UL Output: It is recommended to verify the output of the BDA, which will include Idle UL Noise Output, Minimum Output Signal and Maximum Output Signal. By doing this step, it will validate the required BDA UL output signal, verify the BDA system is not oscillating, prevent any unnecessary noise into the Public Safety network, and follow all FCC rules and guidelines. With the minimum and maximum BDA output signals you can estimate/calculate the minimum and maximum input signals at the donor site. Some AHJ’s require a maximum input signal of -75dBm and a minimum input signal of -95dBm.
BDA Idle UL Noise (Figure 7 below):
Figure 7 – BDA Idle UL Noise Example
Minimum and Maximum UL Output Signals (Figure 8 below):
Most spectrum analyzers have a maximum input power of 10 to 20dBm, and depending on the BDA output power, attenuation at the input of the spectrum analyzer may be required to protect and prevent damage to the spectrum analyzer. Many spectrum analyzers do have an option to offset this external attenuation in their software (check the manufacture spectrum analyzer specifications and/or user manual for details).
Figure 8 – Min & Max UL Output Signal Examples
Simple UL link budget calculations (such as donor cable loss, donor antenna gain and free space pathloss) can be used to determine the minimum and maximum UL signals at the donor site with these measurements.
Even though today’s BDA user interfaces provide some sort of RF information, Comba recommends using a spectrum analyzer to commission any Public Safety BDA or Fiber DAS solutions. In this document, Comba just showed absolute basic features of spectrum analyzers, as it provides more RF details, and if necessary, a better option when troubleshooting an RF issue. The outlined basic measurements in this document should help commission a BDA in a more accurate and successful way.
In short, don’t “shortcut” and do the job the wrong way, use the correct test equipment and do it the right way!
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